International Patent Application, publication number WO 2010/018133 (SmithKline Beecham Corporation), relates to certain purine derivatives disclosed as inducers of human interferon which may be useful in the treatment of various disorders, for example the treatment of allergic diseases and other inflammatory conditions, for example allergic rhinitis and allergic asthma. One particular purine derivative disclosed in WO 2010/018133 is 6-amino-2-{[(1S)-1-methylbutyl]oxy}-9-[5-(1-piperidinyl)pentyl]-7,9-dihydro-8H-purin-8-one. Co-pending International Patent Application, application number PCT/EP2009/051830 (GlaxoSmithKline LLC), discloses a maleate salt of 6-amino-2-{[(1S)-1-methylbutyl]oxy}-9-[5-(1-piperidinyl)pentyl]-7,9-dihydro-8H-purin-8-one.